The present invention relates to a single-lens reflex electronic photographic camera and, more particularly, to a photometric optical system for such a camera.
An electronic photographic camera is generally so constructed that a light beam coming through the objective from an object to be photographed is directed to a pickup device, which generates an electrical video signal representative of the object image. This video signal is stored in a magnetic disc or a semiconductor memory. Such electronic photographic camera is worthy of attention as a substitute for the conventional photographic camera requiring a chemical emulsion to store the object image.
The pickup device is a so-called self-scanning photoconductive device comprising a plurality of small photoconductive elements arranged in an array and adapted to output their stored electric charges successively. Various types such as MOS and CCD are already well known as self-scanning photoconductive devices. In each case, the respective small photoconductive elements are slightly spaced from one another in the array, so the brightness of the object image can be sampled in successive discrete image elements corresponding to the respective photoconductive elements for conversion into an electric signal. In this process, a false signal can occur as the spatial frequency of the object image increases close to the sampling frequency of the pickup device, and consequently undesired phenomena, such as Moire patterns, appear in the image signal, deteriorating its quality.
To avoid such adverse effects, an optical low-pass filter is commonly employed in the light path for photographing so that the high-frequency component is rejected. Consequently, the light beam coming from the object only enters the pickup device after the unnecessary high-frequency component has been removed. The optical low-pass filter may be, for example, a quartz plate or a phase filter. Such optical low-pass filter is optically effective no matter where the filter is located in the light path for photographing extending from the objective to the pickup device. Accordingly, the filter may be located in front of or behind an image-reflecting mirror adapted to reflect the light beam for photographing towards the viewfinder optical system. When the filter is located in front of the mirror, i.e., immediately behind the objective, however, more space is required to avoid interference between the filter and the top of the mirror during its upward swing. As a result, the photographic camera becomes more bulky. Such arrangement is optically also disadvantageous because the light beam without its high-frequency component incident upon the focusing plate of the viewfinder optical system also makes focusing detection difficult.
This problem is avoided by locating the optical low-pass filter behind the image-reflecting mirror and inserting a compensation filter optically equivalent to the optical low-pass filter immediately in front of the focusing plate of the viewfinder optical system. As a result of the compensation plate, there is no difference in the spherical aberration after transmission through the objective or no difference of focusing between the viewfinder image and the actual photographed image.
In the single-lens reflex electronic photographic camera, no light beam enters into the pickup device except during the photographing operation and a photometric element must be located in the light path of the viewfinder optical system for exposure display during observation of an object to be photographed through the viewfinder and for exposure control. Conventionally, a beam splitter is located behind the focusing plate to divert a portion of the light from the light path or the photometric element is located so as to be optically orientated obliquely of the focusing plate or the viewfinder optical axis so that the photometric element does not obstruct observation of the viewfinder image.